The invention concerns an automatic frequency control arrangement, and in particular an automatic frequency control arrangement for use in a homodyne FSK (frequency shift keying) receiver or transceiver.
Communication systems are known which operate by modulating a carrier according to the binary state of a data signal to be transmitted. In such systems, a binary "1" causes the carrier frequency to be shifted by a certain amount in one direction, while a binary "0" causes normally an equal shift in the opposite direction. For this reason, the technique is known as "frequency shift keying" (FSK) and when the shifts are the same in both directions, "symmetrical frequency shift keying".
The local oscillator of a receiver in such an FSK system normally operates at the same frequency as the transmitter carrier, with the result that the signal at the output of the receiver mixer is at baseband IF frequency. This technique is known as "direct conversion" or "homodyne conversion". In an ideal system, where the local oscillator and transmitting oscillator frequencies are the same and the carrier is modulated symmetrically, i.e. is modulated plus and minus, respectively, by equal amounts for the two data states, the mixer output should always be at the same frequency, the baseband frequency, regardless of whether a "1" or a "0" is being received. There will, however, be a phase difference between the I and the Q channels due to the opposing signs of the modulating frequencies for the two data states, and it is this phase difference which is processed in the four-phase detector 44 to reconstitute the transmitted data.
In a practical system, however, there may well be a difference between the carrier and local oscillator frequencies due to drifts in either or both of the oscillators. This drift may be due to various factors such as temperature variations, ageing, etc. When this occurs, the received IF frequency for a "1" state will be different from that for a "0" state and as a result the four-phase detector may be unable reliably to extract the transmitted data; indeed, when drift is excessive data may be totally lost.
It is an aim of the invention to provide an automatic frequency control arrangement which overcomes the above drawback.